My bag is constantly packed, so it's slightly different to listing things i always remember to take/pack.

But my standard kit is very basic toiletries, tissues/toilet paper (you only ever get caught out the hard way once), sheep sleep bag, pillow case, a couple of pens/paper or notepad, phone charger, small padlocks, maglight, earphones (although my tiny radio is dead and needs replacing). Also a folded cloth sack of the type normally used to keep a sleeping bag in/carrier bags, a couple of cycling toe straps, two 330ml water bottles (ex coke type, but of the european returnable style so they are of decent quality, but can easily be replaced as and when they wear out), and a European Thomas Cook TT. All that is always kept in a bag by my door or, often, in the office.

To that I'll add a change of clothes (plus extra underwear/socks), a paperback of some description if i get chance and sometimes a travel towel, plus some cans of beer when opportunity presents itself. In addition, i always have a swiss army knife (even if just going upstairs) and rope with me.

Which (ignoring beer) all gives me a 20l bag roughly 2/3rds full, and will do me for as long as it needs to, more or less.